In a manufacturing process of a semiconductor device or a FPD (Flat Panel Display), plasma is used to perform a process, such as etching, deposition, oxidation or sputtering, so as to perform a good reaction of a processing gas at a relatively low temperature. Conventionally, plasma generated by a high frequency electric discharge in MHz frequency band has been used in this kind of plasma process. The plasma generated by the high frequency electric discharge is largely divided into capacitively coupled plasma and inductively coupled plasma according to a plasma generation method (in view of an apparatus).
Generally, in an inductively coupled plasma processing apparatus, at least a part (for example, a ceiling) of walls of a processing chamber may have a dielectric window, and a high frequency power is supplied to a coil-shaped RF antenna positioned at an outside of this dielectric window. The processing chamber serves as a depressurizable vacuum chamber, and a target substrate (for example, a semiconductor wafer and a glass substrate) is provided at a central region within the chamber. A processing gas is supplied into a processing space formed between the dielectric window and the substrate. A RF magnetic field is generated around a RF antenna by a RF current flowing through the RF antenna. Magnetic force lines of the RF magnetic field penetrate the dielectric window, and pass through the processing space within the chamber. As the RF magnetic field changes with time, an inductive electric field is generated in an azimuth direction within the processing space. Then, electrons accelerated by this inductive electromagnetic field in the azimuth direction collide with molecules or atoms of the processing gas so as to be ionized. In this process, a donut-shaped plasma may be generated.
Since a large processing space is formed within the chamber, the donut-shaped plasma can be diffused efficiently in all directions (particularly, in a radial direction) and a plasma density on the substrate becomes very uniform. However, only with a conventional RF antenna, the plasma density on a substrate is not sufficiently uniform for most plasma processes. As for an inductively coupled plasma processing apparatus, it is also one of the most important issues to improve uniformity of a plasma density on a substrate since a uniformity/reproducibility and a production yield of a plasma process depend on the plasma uniformity. Until now, there have been suggested several techniques related to this problem.
As a representative technique for controlling (particularly, uniformizing) a plasma density distribution on a target substrate in a diametric direction in an inductively coupled plasma processing apparatus, a following method has been well known. That is, a RF antenna is divided into multiple coil-shaped antenna segments and an impedance of each antenna segment is varied by an impedance control circuit. Therefore, a split ratio of a RF power distributed to each of all antenna segments from a single high frequency power supply is controlled (for example, Patent Documents 1, 2 and 3).
Generally, this kind of impedance control circuit includes a variable capacitor and is connected in series to each of multiple antenna segments. Further, the impedance control circuits are connected in parallel between an output terminal of a matching unit and a ground potential terminal.
Typically, a matching unit used in the inductively coupled plasma processing apparatus includes an automatic matching device that variably controls a load impedance viewed from the high frequency power supply depending on a variation of a plasma load. If an impedance of the plasma load is varied due to a variation of a pressure during a plasma process, this kind of automatic matching device is configured to vary a reactance of a variable reactance element (generally, a variable capacitor) in a matching circuit. Further, this automatic matching device is configured to automatically control a load impedance to set the load impedance to be a matching point (typically, 50Ω). For this automatic matching function, the automatic matching device includes a circuit that measures a power of a reflection wave or a controller that variably controls a reactance of each variable reactance element by a stepping motor such that a measurement value of the load impedance is matched to the matching point (50Ω).    Patent Document 1: U.S. Pat. No. 6,164,241    Patent Document 2: U.S. Pat. No. 6,288,493    Patent Document 3: U.S. Pat. No. 7,096,819
However, in the above-described conventional RF antenna division method, since the RF current is split to flow through each of the multiple antenna segments, a pre-divided RF current flowing through a high frequency power supply unit is remarkably greater than the RF current flowing through each antenna segment. Therefore, there is a great loss of a RF power within the high frequency power supply unit (particularly, the matching unit), and the RF power to be supplied to the plasma load is decreased as much as the loss. As a result, a plasma generation efficiency is decreased.